When any particular key is pressed, its value will appear in BCD form at the outputs (A, B, C & D). It will remain there until another key is pressed. The 12 keys produce outputs up to `1011`. Extended to 16 keys, the circuit will give the full HEX to BCD conversion.
The above circuit produces an output ONLY while the input switch is depressed. To make a convertor with a latched output, the following modifications are made. Each CMOS `AND` gate has its free input tied to Vcc, and by the action of R1 through